Hot air provides key to complex polymer structures

Sometimes the most complex-seeming things can be simple. Case in point: an ordered 0.2-µm polymer array created by evaporating a polymer solution at room temperature. Mohan Srinivasarao, a polymer chemist at the Georgia Institute of Technology (Atlanta, GA), produces self-assembling hexagonal structures within seconds (see figure). The finished structure consists of interconnected spherical air bubbles ranging in diameter from 0.2 µm to 20 µm. A 30- to 40-µm thick polymer film may contain as many as 15 layers. "This is an easy way of making materials with the regular structure needed for optical and photonic applications," Srinivasarao says.

Although the potential uses for such structures are numerous, Srinivasarao and associates are initially focusing on testing polymers with a refractive index of 2.5 and 2.6 in hopes of developing a photonic bandgap material. "We have not demonstrated their use for photonic bandgaps yet," he says, "but we believe it is possible." The researchers also are incorporating laser dyes with the polymer solution to study the potential of lasing the dye molecules in such a structured medium.

Srinivasarao forms the structure by dissolving a coil-like polymer such as polystyrene in a fast-evaporating solvent such as toluene or benzene. He places the solution containing the dilute (0.1% to 5% by weight) polymer on a glass slide and directs moist air across it as the solvent evaporates.

Rapid evaporation of the solvent lowers the temperature of the solution by 25°C. Moisture from the warmer air condenses on the surface of the solution, forming a layer of uniform-size droplets. Because the water is denser than the solvent, the layer of droplets sinks into the sample, allowing another layer to quickly form on top of it. The process repeats itself for one to two minutes until the solvent evaporates, producing a three-dimensional pattern of closely packed water droplets preserved in the polymer film. Then the water evaporates to leave an array of bubbles.

Why does this work? "It is such a simple process, but it is very complicated to explain," says Srinivasarao. Under normal conditions, a water droplet readily joins with another. "But in our case, they do not seem to recognize each other," he says. Srinivasarao believes that heat liberated through the condensation process makes the surrounding air hotter than that directly above the solution. This creates a convective flow within the droplets, which is enough to keep them apart. "I may be wrong," he says, "but I'm convinced that I'm not."

The National Science Foundation awarded Srinivasarao a five-year Career Grant for his work. Andrew Lovinger, director of the polymer program of the National Science Foundation, sees great potential in the research. "This is still very fundamental research, but Mohan's technique is so easy to realize. It requires no high pressure, vacuums, or other high-tech equipment, and the process is finished in a few seconds," Lovinger says. "A number of people should be eager to jump in and experiment with this technique."

Other applications include the development of optical switches, membranes, filters, waveguides, antennas, solar cells, and various biomedical treatments.